This invention relates generally to the production of extreme ultraviolet radiation and soft x-rays and particularly to a discharge source apparatus for generating extreme ultraviolet radiation for projection lithography.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths. Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only when serving several steppers.
Another source is the laser plasma source (LPS), which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. LPS is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). The plasma is produced by a high-power, pulsed laser that is focused on a metal surface or in a gas jet. (See, Kubiak et al., U.S. Pat. No. 5,577,092 for a LPS design.)
Discharge plasma sources have been proposed for photolithography. Capillary discharge sources have the potential advantages that they can be simpler in design than both synchrotrons and LPS""s, and that they are far more cost effective. Klosner et al., xe2x80x9cIntense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography,xe2x80x9d Opt. Lett. 22, 34 (1997), reported an intense lithium discharge plasma source created within a lithium hydride (LiH) capillary in which doubly ionized lithium is the radiating species. The source generated narrow-band EUV emission at 13.5 nm from the 2-1 transition in the hydrogen-like lithium ions. However, the source suffered from a short lifetime (approximately 25-50 shots) owing to breakage of the LiH capillary.
Another source is the pulsed capillary discharge source described in Silfvast, U.S. Pat. No. 5,499,282, which promised to be significantly less expensive and far more efficient than the laser plasma source. However, the discharge source also ejects debris that is eroded from the capillary bore and electrodes. An improved version of the capillary discharge source covering operating conditions for the pulsed capillary discharge lamp that purportedly mitigated against capillary bore erosion is described in Silfvast, U.S. Pat. No. 6,031,241.
Debris generation and high-power operation remain two of the most significant impediments to the successful development of the capillary plasma discharge sources in photolithography. Debris generated by the capillary tends to coat optics used to collect the EUV light which severely affects their EUV reflectance. High power is required to achieve adequate wafer throughput and low cost of ownership. Ultimately, this will reduce their efficiency to a point where they must to be replaced more often than is economically feasible. The art is in search of EUV radiation sources that do not generate significant amounts of debris.
The invention is based in part on the recognition that using of several discharge sources that are multiplexed together in time can significantly reduce the amount of debris generated. It is expected that with inventive radiation source, the peak discharge source temperature will be lower than it would be if a single discharge source were used in continuous operation so the debris production will be less.
In one embodiment, the invention is directed to an illumination system that includes:
(a) a first electromagnetic radiation source array that includes a plurality of first activatable radiation source elements that are positioned on a first movable carriage;
(b) a second electromagnetic radiation source array that includes a plurality of second activatable radiation source elements that are positioned on a second movable carriage;
(c) means for directing electromagnetic radiation from the first electromagnetic radiation source array and electromagnetic radiation from the second electromagnetic radiation source array toward a common optical path; and
(d) means for synchronizing (i) the movements of the first movable carriage and of the second movable carriage and (ii) the activation of the first electromagnetic radiation source array and of the second electromagnetic radiation source array to provide an essentially continuous illumination of electromagnetic radiation along the common optical path.